A doppler sensor for detecting a human body has been provided in the prior art (see, e.g., Japanese Patent Application Publication No. 2010-191857. The doppler sensor is designed to transmit or radiate an electric wave (transmission wave), e.g., millimeter wave, to a detection range and receive electric waves (reflected waves) reflected from the detection range, thereby obtaining a doppler signal of a frequency band corresponding to a movement speed of a human body and determining whether or not a human body exists within the detection range depending on the doppler signal.
Another sensor for detecting a human body is, for example, a thermal sensor for detecting thermic rays radiated from a human body. However, in comparison to this thermal sensor, the doppler sensor has the advantage of detecting a human body located at a long distance. Accordingly, the doppler sensor is more adequate for the case of being fixed to a high ceiling in a building.
The doppler sensor as described above is used in an illumination device which turns on and off a light source depending on a detection of a human body by the doppler sensor. In comparison to the case when the light source is manually turned on and off, such an illumination device makes it possible to reduce unnecessary power consumption caused when a user forgets manipulation of turning off the light source.
In the prior art doppler sensor, the sum of intensities of a certain number of frequencies in a doppler signal is compared with a certain threshold value and, when the sum is higher than the threshold value, it is determined that a human body exists.
However, it happens that, even with moving objects, other than a human body, the sum can be higher than the threshold value, causing erroneous detection.